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The effect of the type of sugar used as substrate on the susceptibility of Streptococcus mutans to antibiotics was evaluated. Thirty strains, grown in excess of sucrose (s-MIC) and in excess of glucose (g-MIC), were tested for susceptibility to four cephalosporins. About 21% of the strains were sensitive in the presence of sucrose and resistant in the presence of glucose, whereas only 3% of the strains showed the opposite situation. The mean values of the s-MICs of the four cephalosporins were significantly lower than those of the g-MICs. These findings may also be explained by the synthesis, in excess of sucrose, of insoluble glucan by Streptococcus mutans which increases the interbacterial distance and promotes antibiotic diffusion. Given the susceptibility of Streptococcus mutans grown in excess of sucrose to cefotaxime and cefepime, these antibiotics may be used in the primary prevention of infective endocarditis, when subjects predisposed to endocarditis need invasive dental therapy.

It has been clearly established that the inoculum size greatly affects the results of antibiotic susceptibility tests performed in both liquid and solid media in standard laboratory growth conditions (i.e. planktonic). Recently methods were developed to perform antibiotic susceptibility tests on bacteria growing in sessile conditions. The present study investigates the effect of the inoculum size on results obtained by these methods. Results show that the inoculum size does not affect tests performed in sessile conditions. A simple and reliable method is proposed to be applied to routine microbiological laboratory procedures.

The validity of nonsimultaneous association of penicillinase-sensitive beta-lactam antibiotics with halogenated isoxazolylpenicillins was tested in various Enterobacteriaceae strains. With several bacterial strains, expecially if producers of beta-lactamase type IV, exposure of either the bacterial suspensions of the brothsupernatant to the isoxazolylpenicillins appears both in vivo and in vitro to reduce the extent of subsequent hydrolysis of penicillinase-sensitive beta-lactam antibiotics.

The antibacterial activity of 6 antibiotics towards 10 gram-positive and 6 gram-negative glycocalyx-producing strains, has been evaluated by employing a method which partially simulates the in vivo colonization of prosthetic devices. The results showed that routine antibiotic sensitivity tests are not predictive about the response of the glycocalyx-embedded bacteria, and that prophylaxis may be useful with ofloxacin and clindamycin, before placing a prosthetic device. Once bacterial colonization had already occurred, however, none of the tested antibiotics was able to eradicate the sessile bacterial form. The minimum bactericidal concentration (MBC) values, indeed, were much higher than those determined on the planktonic form, and were much higher than serum and tissue levels that can be reached in vivo.

We found that nalidixic acid is a good inhibitor of conjugal transfer of R-plasmids and that related compounds show the same properties. We investigated recently synthetized quinolinic molecules. The inhibition was not due to bacterial activity of the compounds against donors, recipients or R transconjugants; in fact, the drug concentrations were twice or four times lower than the M.I.C. values. The new molecules showed a marked inhibitory effect in various R-mating experiments.